1. Field of the Invention
The present invention relates generally to the design of bits used for drilling bores through earthen formations. More particularly, the present invention relates to a method for optimizing the creation of a well bore using a bit designed to operate with a specified drilling system, formation structure and well bore trajectory.
2. Setting of the Invention
The prior art monitors real-time drilling conditions to optimize drilling effectiveness. Generally, the prior art method for optimizing drilling performance has not gone as far as designing an optimum bit for the given drilling system and formation requirements. The prior art has not attempted to optimize the performance of a bit by designing the bit to be most effective operating within the specific drilling parameters of a particular well. In the inventor's experience, prior art bit selection processes have not taken into account information about the drilling system and the formation of the drilling application and then proceeded to design a bit to work optimally within the parameters of the application. A comprehensive technical approach to handling bit selection is not generally taken. Bit selection for a particular application is generally done intuitively based on general rules of thumb, but not on a scientific or technical basis.
For example, in selecting a bit design for a directional application, the bit may be selected based on the general knowledge that small cutters perform better in a directional application. The specific details as to the reasons for, or extent of, the improved performance are not generally considered. It is generally known that certain bit profiles steer better; it is known that certain cutter shapes steer better; and it is known that back rake can affect steering performance.
Generally speaking, when a bit selection is made, the selection is based on a combination of these intuitive reactions as well as empirical data that is obtained in the past from using certain bits in these environments. In selecting a bit, certain parameters regarding the drilling system and formation are also generally known, such as the motor torque requirements and performance, the motor RPM the range of compressive strength in the formation, and the range of ROP for a particular bit design. Using these parameters, an intuitive analysis is made where, for example, the number of blades needed is determined, the cutter sizes are determined, and the bit profile is determined. The approach has generally been purely intuitive. With the current bit selection process, twenty different bit designers may come up with twenty different combinations in a bit to achieve the final design.
In most drilling situations, it is usually desirable for the bit to achieve the maximum possible rate of penetration (ROP) through the formation. The rate at which the drill bit penetrates the formation is determined primarily by the formation characteristics, the design of the drill bit, and the characteristics of the drilling system used to drive the bit.
Some drilling systems, such as, for example, those that are employed to drill directional wells, drive the bit with a subsurface motor. The speed of rotation and the torque output of the motor are examples of operating characteristics of the motor itself that affect the drill bit ROP.
When the reaction torque of the bit against the formation exceeds the output torque capability of the motor, the bit will cease rotation. A number of factors affect the reaction torque value. These include the hardness, or compressive strength of the formation, the configuration of the engaging interface between the bit and the formation, and the magnitude of the forces driving the bit into the base of the bore hole. Of these factors, the weight on bit (WOB), which supplies the primary force driving the bit against the base of the well bore, is one of the most easily measured forces affecting the drilling process, and it is a force that can be conveniently monitored at the well surface during the drilling process.
With the bit off the bottom of the well bore, a condition in which there is essentially no weight on the bit, the reaction torque of the rotating drill bit is very small. As weight is applied to the bit, the reaction torque increases until the weight exceeds a value at which the speed of rotation and torque output begin to decrease. The relationship between the torque output of the bit and the WOB can be established for a given drill bit size and bit design operating in a specified formation.
In practice, the torque versus WOB curve for a specific bit is the most frequently employed guide to determine the operating conditions for the bit. Thus, where the maximum torque output from the bit is desired, the WOB is maintained at the level indicated by the curve to produce this maximum torque. Generally, maintaining maximum torque output at the bit produces the best ROP. This is not, however, always the case, and maintaining the maximum allowable weight on the bit does not necessarily produce the maximum torque in the bit.
The practice of using WOB to determine torque is only indirectly related to the primary objective of optimizing ROP. Maintaining an optimum output torque on the bit of a particular bit design does not necessarily produce the optimum ROP capable of being attained with the drilling system and well formation of the application.